1. Field of the Invention
The invention relates to an arrangement for increasing the stability in high-voltage networks.
2. The Prior Art
In order to increase the stability of high-voltage electric power supply networks or in order to be able to increase the transmitting capability of a power line while maintaining the stability, series capacitors are used, which reduce the inductive line reactance, thus making the lines electrically shorter and stronger. However, arranging series capacitors involves the complication that, as a consequence of overcurrents and transients in the case of line faults, the capacitor will be subjected to overvoltages which may damage and completely destroy it and against which it must be protected. Such a protective device comprises a spark gap which ignites instantaneously and short-circuits the capacitor when the protection level determined by the ignition voltage of the spark gap has been reached. This means that the compensating effect of the series capacitor is lost at the moment it is most needed, that is, during the occurrence of the fault itself. Furthermore, the protective gaps require a certain time for de-ionization after they have been extinguished, which delays the reinsertion of the capacitor.
If the capacitor is dimensioned so that during normal operational conditions it compensates the line reactance with a sufficient margin, it may however happen that a disturbance, which has caused a disconnection of the capacitor, is so strong that the stability is lost during the period for which the capacitor has been disconnected and that the capacitor is not able to restore the stability after reconnection. In order to eliminate such a risk, the capacitor is made so large that it may safely restore the stability after connection. This arrangement does of course involve a corresponding increase in the cost of installation of the capacitor and is therefore made necessary by the demand that the transmission stability should always be assured.
The protective spark gaps which are used for short-circuiting series capacitors are of essentially two kinds, self-extinguishing and non-self-extinguishing gaps. Non-self-extinguishing gaps require parallel connections with current transformers, relays and circuit-breakers in such a way that, when the gap is ignited, a circuit breaker is closed and short-circuits the gap which then expires. When the gap has been de-ionized, which is a question of statistical nature, the breaker is to open and restore the capacitor. Considering, among other things, the fact that the use of non-self-extinguishing gaps requires breakers for extinguishing the gap and restoration of the capacitor and the fact that the breaker is the weakest link in a power system, there is always a certain risk that the restoration of the capacitor may fail and that the stability is lost, which may involve considerable damage and expense. Self-extinguishing gaps, with or without additional elements, have the ability of being able to expire a short time after the fault has been disconnected and the line current has decreased to the normal value. In this way there is generally no need of parallel-working devices such as circuit breakers, which must be regarded as an important advantage.
One of the disadvantages of these gaps, however, is that for certain types of faults occurring within certain lines it must be assumed that the gap is ignited and extinguished each half-period at the zero-passage of the current, in which case the capacitor is discharged each time with a frequency which is many times higher than the power frequency. For such cases the capacitor must be able to withstand a higher voltage, which increases the cost.
In capacitor banks, known up to the present, for maintaining the stability by compensation with series capacitors protected by self-extinguishing or non-self-extinguishing gaps, the great disadvantage remains that the stabilization is interrupted for a considerable length of time when an over-voltage occurs in the capacitor at the very moment when the stability is highly stressed.